Refactor SystemPCIeDevice method

Move SystemPCIeDevice to a separate method, and refactor the code.
Validate the PCIe device path.

Tested: Validator passed

```# curl -k https://$bmc/redfish/v1/Systems/system/PCIeDevices/pcie_card7


{
  "@odata.id": "/redfish/v1/Systems/system/PCIeDevices/pcie_card7",
  "@odata.type": "#PCIeDevice.v1_9_0.PCIeDevice",
  "Id": "pcie_card7",
  "Name": "PCIe Device",
  "PCIeInterface": {
    "LanesInUse": 4,
    "PCIeType": "Gen1"
  }
}
```

Change-Id: Ib9bd3ff37293b64adbf9987c3d932882befd21d4
Signed-off-by: Lakshmi Yadlapati <lakshmiy@us.ibm.com>